Supporters of the two candidates in Colombia's presidential run-off flooded the Internet on Saturday with urgent appeals for voters to cast ballots in an election that could suffer from low turnout.

Former Defense Minister Juan Manuel Santos won a first round in May and holds a commanding lead in opinion polls ahead of Sunday's contest against ex-Bogota mayor Antanas Mockus, the Green Party candidate who has promised to stamp out corruption.

The winner will take over from President Alvaro Uribe, a conservative U.S. ally who battered leftist FARC guerrillas and nurtured booming foreign investment in the Andean country once notorious for its bombings, kidnappings and drug barons.

Santos, who is touting his experience fighting the rebels and creating jobs, used his Facebook page to urge Colombians to choose the path of democratic security, sovereignty, freedom, justice and peace.

Analysts say turnout could fall because of his big lead in the polls, and the World Cup.

Everyone here, together with millions more, we have no excuse not to vote (Sunday) ... Colombia is not in the Cup! But we could lose so much, so go out and vote for President Santos! read one of hundreds of comments from supporters.

Sunday's winner inherits a country that is much safer than when Uribe came to power in 2002, but will have to tackle Latin America's highest unemployment, a stubborn fiscal deficit and fractious ties with neighboring Venezuela where a trade dispute is weighing on Colombia's economic recovery.

Investors broadly view a victory by Santos as a continuation of Uribe's security and pro-business policies, and say a Santos win will maintain a favorable support in the short term for the peso currency and local TES bonds.

The latest Invamer-Gallup opinion poll said Santos should take 66.5 percent of votes against 27.4 percent for Mockus.

LAST-MINUTE APPEALS

Mockus, who had tapped into discontent over scandals linked to misuse of state funds and charges of abuses by the security forces, received only half the votes Santos got in the May first round -- but enough to stop his rival from winning outright.

On his Facebook page, the tone of the comments was urgent: Social networking colleagues, send messages to your friends, to those abstaining, undecided or members of other parties. Add videos and links to argue our position, now! Mockus for president! read one appeal.

Mockus admits voters are wary of his frank positions on issues including taxes, which he wants to raise to ease the deficit. But as mayor he was known for his fiscal discipline.

Once written off as a failing state, Latin America's No. 4 oil producer has seen its long war ebb as Uribe used billions of dollars of U.S. aid to send troops backed by helicopters and better military intelligence to drive back the rebels.

With the country's economy slowly pulling back from the global crisis, polls show voters now want the new government to tackle a 12.4 percent unemployment rate, a large informal economy and a public healthcare system.

The fiscal deficit -- seen at 4.4 percent of GDP in 2010 -- is also a barrier that Wall Street sees to Colombia regaining investment grade on its sovereign debt. Both candidates have said they will increase tax revenue to fight the deficit.

(Editing by Jack Kimball and Xavier Briand)

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